Honing tool

ABSTRACT

A honing tool, in particular an internally or externally toothed honing ring for the finish machining of gears, containing a synthetic-resin bonding-agent matrix ( 1 ) and ceramically bonded abrasive-grain hollow bodies ( 2 ) embedded therein.

[0001] The invention relates to a honing tool. It relates in particular to a toothed honing tool for the finish machining of gears. The engagement or contact roll honing of gears by means of correspondingly toothed honing rings is known (U.S. Pat. No. 2,105,896). Furthermore, toothed honing tools are known (EP-A 692 342, DE-C 44 47 036) which contain abrasive grain conglomerates ceramically bonded in a synthetic resin matrix. They permit greater material removal than conventional honing tools, in which the abrasive grains are in each case embedded individually in a synthetic resin matrix, so that, if need be, a special grinding operation before the honing can be dispensed with. This is due to the fact that the grains, owing to the ceramic bond, are rigidly held in the conglomerates and can therefore be brought into effect on the workpiece with greater rigidity than is possible with grains held individually in the synthetic resin matrix. Although the conglomerates are embedded in the softer synthetic resin matrix, they are scarcely able to yield under the cutting forces because the force transmission between them and the matrix takes place via their surface; which is very large compared with the surface of an individual grain. In this respect, the abrasive properties depend, inter alia, on the size of the conglomerates and thus on the quantity of the ceramically bonded abrasive grain and also on the quantity of the ceramic bonding agent, this quantity restricting the freedom in adjusting the abrasive properties.

[0002] The object of the invention is to improve the adjustability of the abrasive properties. It achieves this by the features of claim 1 and preferably by those of the subclaims.

[0003] Accordingly, a honing tool contains hollow bodies which are embedded in a matrix of softer bonding agent and have abrasive grain bonded by a harder bond. These mechanical properties of the matrix bonding agent or of the hollow body bond are to a considerable extent formed by their moduli of elasticity. The ratio of the modulus of elasticity of the material of the harder bond to that of the softer bonding agent is to be at least 2:1, preferably at least 4:1. This corresponds to the ratio of the moduli of elasticity of epoxy resins and ceramics currently used as bonding agents for grinding tools. An organic bonding agent is therefore expediently used for the matrix, whereas ceramic substances are expediently used for the bond of the hollow bodies.

[0004] The abrasive-grain hollow bodies according to the invention each have greater rigidity on their own than the surrounding matrix material. They consequently behave like the previously known conglomerates, since, firstly, they are supported on the matrix over a large area via their surface and, secondly, owing to the firm bond, they give the abrasive grains contained in them the hold required for a high specific material removal capacity. However, whereas in those conglomerates the size of the conglomerate area acting in the abrasive area on the tool in a chip-removing and abrading manner is in a fixed ratio to the overall size of said conglomerate, the size of these areas, owing to the invention, can be set as desired by determining the size of the cavity. The abrasive force which is transmitted via an abrasive-grain hollow body in a tool according to the invention acts on a smaller number of abrasive grains than in a conglomerate of the same size of a known honing tool. When an abrasive force which is the same overall is applied, a larger specific abrasive force is therefore transmitted to the individual abrasive grain and a correspondingly higher abrasive capacity is produced by this grain. If it is desired to achieve the same specific abrasive force at the abrasive grain or the same grain-related abrasive capacity as in the prior art, this is possible with a lower abrasive force overall in the honing tool according to the invention. In addition, the frictional forces between tool and workpiece are lower, since, firstly, the number of abrasive grains which are in engagement is smaller and, secondly, the proportional area of the bonding material located between them in the abrasive area is smaller. The greater material removal desired can already be achieved when using smaller abrasive-grain hollow bodies. The risk of the tool being damaged by the cutting and normal forces which occur during the honing is thus further reduced. Furthermore, the machining quality can be increased as a result.

[0005] Further advantages of the invention consist in the fact that the mechanical and thermal loading of tool and workpiece during the machining can be reduced. Accordingly, better reproducibility of the working result can be achieved. The regular arrangement of cavities in the tool close to the grain permits improved discharge of the honing dust from the engagement zone and improved feeding of auxiliary substances into the engagement zone.

[0006] The hollow bodies are expediently spherical, since this shape promises optimum strength. In addition, a geometrically largely predetermined arrangement of the abrasive grains and thus improved reproducibility of the tool and of the working result are obtained.

[0007] It is known (DE-C 23 483 38, DE-A-2350139, DE-A-2410808, DE-A-2951067) to use abrasive-grain hollow balls for producing grinding and polishing tools. However, they have only proved successful in flexible grinding tools (grinding belts). Although it has also been proposed to produce rigidly bonded abrasive bodies (grinding wheels) therefrom (U.S. Pat. No. 3,928,949), these abrasive bodies did not achieve the capacity of conventional grinding wheels. It has surprisingly been found that this disadvantage does not occur during use in honing tools. This is probably accounted for by the different stress. Abrasive operations are characterized by a high relative velocity between the workpiece and the tool, which lead to high thermal loading of the tool surface. It was thought that, in addition to other advantages, a higher thermal resistance of the grinding tools could be achieved through the use of hollow balls (DE-A-2951067). On the other hand, the relative velocity during honing is low and the influence of the pressure between tool and workpiece is substantially greater than during grinding.

[0008] According to a special feature of the invention, the hollow bodies consist of a supporting shell which is arranged on the inside and on whose outer surface the abrasive grain is anchored by means of a bonding agent. Suitable supporting shells are known (U.S. Pat. No. 4,111,713). The bond of the abrasive grains belonging in each case to a hollow body is formed on the one hand by the supporting shell and on the other hand by the bonding agent acting between the supporting shell and the abrasive grains. The required higher rigidity of the hollow bodies compared with the matrix material comes about by both the supporting shell and the bonding agent expediently being made of a material having a high modulus of elasticity. However, it is also possible to merely provide the supporting shell with the desired rigidity (high modulus of elasticity) and to use a softer bonding agent, and vice versa. The presence of a special supporting shell has the advantage that the stability of the hollow body does not have to be produced solely by the bonding agent. The latter can therefore be used more sparingly in the grain vicinity, so that the grains can come into engagement in a freer manner without enveloping bonding agent having to be abraded beforehand. In this case it is possible to select a material for the supporting shell which, although it imparts the requisite strength, can be abraded easily in order to produce the lowest possible frictional forces. Ceramic materials are also especially suitable for this.

[0009] Very expedient is the use of such abrasive-grain hollow balls as are currently found on the market as HERMESIT® in flexible abrasive agents on a backing, to be precise preferably that variant of HERMESIT® which has hollow balls with a supporting shell of ceramic material, on the outer surface of which supporting shell the abrasive grain is bonded in a single-layer arrangement with a ceramic bonding agent.

[0010] The average outside diameter of the hollow bodies is expediently equal to one third to one twentieth, preferably one quarter to one tenth, of the tooth height. Furthermore, their average outside diameter is expediently between five and fifty times, preferably five and twenty-five times, the average grain size. The thickness of the grain layer should not be greater than five times the average grain size. Hollow balls are preferred in which the supporting shell is surrounded by no more than two layers, preferably by only one layer, of abrasive grain, which corresponds to a ratio of the average grain size to the layer thickness of 1:1 to 1:2.5. The volumetric proportion of the abrasive-grain hollow bodies (including their cavity volume) is expediently between 20 and 70% of the total volume. The pore space present is expediently restricted essentially to their cavities.

[0011] The absolute grain hollow-body sizes, depending on the grain, lie as a rule between 800 and 3000 μm outside diameter. The possible grain size range extends from P 40 to P 600. This corresponds to an average grain size of 412 μm to 26 μm. The grain size range from P 100 to P 320, corresponding to 156 μm to 46 μm, is preferred for gear-production honing.

[0012] Depending on the size of the conglomerates, the cavity diameter preferably lies between 600 and 1800 μm. The ratio of the average grain size to the wall thickness expediently lies between 1:1 and 1:5, and is preferably between 1:1.5 to 1:2.5.

[0013] In principle any material which is suitable for the application may be used for the abrasive grain, e.g. fused alumina, ceramic corundum, silicon carbide. The use of high-performance abrasive agents such as cubic boron nitride is also suitable.

[0014] The synthetic resin matrix expediently contains additional abrasive grain, it being possible for this to be present in the form of individual grains or smaller conglomerates.

[0015] The abrasive grain bonded in the synthetic resin matrix in addition to the abrasive-grain hollow bodies may be of the same type as that used in the hollow bodies or also of a different type. The grain size is preferably the same. However, it may also differ. Depending on the application and machining purpose, the proportion of the additional abrasive grain bonded in the synthetic resin matrix may be greater, but also smaller, than the proportion of the abrasive grain ceramically bonded in the hollow bodies.

[0016] From one point of view, this additional abrasive grain serves as a hard filler in the synthetic resin matrix and thus strengthens the support of the ceramically bonded walls of the hollow bodies. From a second point of view, this additional abrasive grain itself participates in the machining process. In this case, due to its comparatively more elastic bond, it improves the surface quality which can be achieved.

[0017] All suitable synthetic resins are suitable for the synthetic resin matrix, in particular those which have already proved successful in honing tool production, bonding agents on an epoxy resin basis being preferred. The honing tools according to the invention have a markedly more efficient machining behavior than exclusively synthetic-resin-bonded honing rings without at the same time suffering from the sensibility to brittle fracture of purely ceramic honing tools.

[0018] Compared with previously known honing rings, the tools according to the invention have in particular the following advantages. The cutting forces are lower. The thermal loading of the workpiece is also correspondingly lower. The cavities of the opened abrasive-grain hollow bodies act on the surface of the tool as large pores which receive chips and abrasive dust and are kept away from the direct contact zone between workpiece and tool. The abrasive grains need not be enveloped completely by a ceramic bond and therefore do not need to first cut free before they come into use. This applies in particular if they are bonded merely by their anchoring on a supporting shell. As a result, the frictional forces between workpiece and tool and the thermal loads originating therefrom are reduced considerably. The large cavities permit improved transport of auxiliary cutting substances, such as cooling lubricants for example. During the manufacture of tools, the essentially spherical form of the hollow bodies considerably facilitates the mixing of the components compared with ceramic fragments or conglomerates of an undefined form. Finally, it is a great advantage that, for the provision of the hollow bodies, recourse may be had to a semifinished product which is already available in a large quantity which can be produced economically, namely the abovementioned HERMESIT® abrasive-grain hollow balls.

[0019] The drawing illustrates the structural build-up of a honing tool. Abrasive-grain hollow balls 2 which each contain an essentially spherical cavity 3 are embedded in a synthetic resin matrix 1. In the layer 7, the abrasive grains 4 are connected to one another and if need be to a ceramic supporting shell 5 by means of a ceramic bonding agent. Furthermore, individual grains 6 are contained in the matrix 1 in a uniformly distributed manner. The matrix is essentially free of pores.

[0020] The honing tool according to the invention is preferably used in the form of a honing ring for honing the tooth flanks of a gear. In this case, the tooth profiles of the gear and of the honing ring roll on one another. At the same time, an oscillating relative movement takes place in the axial direction as a rule. The rotation axes of the gear and of the tool may deviate from the parallel position. The gear and/or the honing ring may be toothed on the outside or inside. The relative velocity in the respective contact region is expediently below 5 m/sec and above 1 m/sec.

EXAMPLE

[0021] For the manufacture of a honing ring according to the invention having the dimensions 300×40×200 mm, a total raw material mixture of 4125 g is provided. The latter is composed as follows:

[0022] HERMESIT RE/AK Grain P 120 32.75% by weight

[0023] (Ball outside diameter 800 to 1800 μm, average grain size about 120 μm, average wall thickness about 300 μm)

[0024] Special white fused alumina F 100 22.63% by weight

[0025] Fused alumina F 120 22.63% by weight

[0026] Bond:

[0027] Commercially available epoxy resin 21.99% by weight

[0028] After the constituents have been weighed, first of all grain and bond are mixed for 20 minutes in an agitator mixer at 25 rev/min. The ceramic grain hollow balls (HERMESIT) are then added and mixed with the rest of the mass for 2 minutes at 180 rev/min, so that a homogeneous mixture overall is obtained.

[0029] A four-part mold (plate, rim, core, ring) is cleaned, spread with release agent and fixed on a rotary table. The homogeneous mass is poured in uniformly and spread smooth. After the mold is closed, it is preheated in a preheating furnace for 35 minutes at a furnace temperature of 250° C. The mold is then fitted into a hot press and is pressed at a nominal temperature of 180° C. with a pressure of 8 Mpa for 35 minutes.

[0030] After the cooling, the prehardened honing ring is removed from the mold and is finish-hardened at 165° for 6 hours. The hardened ring is turned to size and toothed. 

1. A honing tool which contains hollow bodies (2) which are embedded in a matrix (1) of softer bonding agent and consist of abrasive grain bonded by a harder bond.
 2. The honing tool as claimed in claim 1, characterized in that it is a toothed honing ring for the finish machining of gears.
 3. The honing tool as claimed in claim 1 or 2, characterized in that the ratio of the moduli of elasticity of the harder bonding agent and of the softer bonding agent is at least 2:1.
 4. The honing tool as claimed in one of claims 1 to 3, characterized in that the harder bonding agent is of a ceramic type and the softer bonding agent is of an organic type.
 5. The honing tool as claimed in one of claims 1 to 4, characterized in that the hollow bodies (2) are spherical.
 6. The honing tool as claimed in one of claims 1 to 5, characterized in that the bond of the hollow bodies consists of a supporting shell (5) and a bonding agent bonding the abrasive grain (4) at its outer surface, and at least the shell (5) or the bonding agent is harder than the bonding agent of the matrix (1).
 7. The honing tool as claimed in one of claims 1 to 6, characterized in that the thickness of the grain layer (7) of the hollow bodies is on average not greater than five times the average grain size.
 8. The honing tool as claimed in one of claims 1 to 7, characterized in that the grain layer (7) has essentially only the thickness of an abrasive grain (4).
 9. The honing tool as claimed in one of claims 1 to 8, characterized in that the average outside diameter of the hollow bodies (2) is equal to five to twenty-five times the average abrasive grain size.
 10. The honing tool as claimed in one of claims 1 to 9, characterized in that the average outside diameter of the hollow bodies (2) is equal to one third to one twentieth of the tooth height.
 11. The honing tool as claimed in one of claims 1 to 10, characterized in that the volumetric proportion of the hollow bodies (2) is between 20 and 70% of the total volume of the honing tool.
 12. The honing tool as claimed in one of claims 1 to 11, characterized in that the bonding agent matrix (1) contains additional abrasive grain (6).
 13. The honing tool as claimed in claim 12, characterized in that the abrasive grain type and/or size of the grain (4) bonded in the hollow bodies (2) and of the grain (6) additionally bonded in the matrix (1) are roughly the same.
 14. The honing tool as claimed in one of claims 1 to 12, characterized in that the pore space present is restricted essentially to the cavities of the hollow bodies (2).
 15. A method of honing the flanks of a gear by generating roll motion of a correspondingly toothed honing ring, characterized by the use of a honing ring as claimed in one of claims 2 to
 14. 16. The method as claimed in claim 15, characterized in that the relative velocity between the tooth flanks of the gear to be machined and of the honing ring in the region of their mutual contact is not greater than 5 m/sec. 